A near-infrared non-fullerene electron acceptor for high performance polymer solar cells

Low-bandgap polymers/molecules are an interesting family of semiconductor materials, and have enabled many recent exciting breakthroughs in the field of organic electronics, especially for organic photovoltaics (OPVs). Here, such a low-bandgap (1.43 eV) non-fullerene electron acceptor (BT-IC) bearing a fused 7-heterocyclic ring with absorption edge extending to the near-infrared (NIR) region was specially designed and synthesized. Benefitted from its NIR light harvesting, high performance OPVs were fabricated with medium bandgap polymers (J61 and J71) as donors, showing power conversion efficiencies of 9.6% with J61 and 10.5% with J71 along with extremely low energy loss (0.56 eV for J61 and 0.53 eV for J71). Interestingly, femtosecond transient absorption spectroscopy studies on both systems show that efficient charge generation was observed despite the fact that the highest occupied molecular orbital (HOMO)–HOMO offset (Δ E H ) in the blends was as low as 0.10 eV, suggesting that such a small Δ E H is not a crucial limitation in realizing high performance of NIR non-fullerene based OPVs. Our results indicated that BT-IC is an interesting NIR non-fullerene acceptor with great potential application in tandem/multi-junction, semitransparent, and ternary blend solar cells.